In a conventional lamp, the light generated by means of an incandescent filament is emitted and is available as useful light. The lamp can have a reflector, by means of which the emitted light can be deflected in such a way that it leaves said reflector as a directional beam and is available as useful light. Lamps on the basis of new kinds of light sources such as LEDs (light emitting diodes) or laser diodes are increasingly gaining in importance nowadays. However, only a limited number of light colors can be generated with these new kinds of light sources. In order to extend this selection, luminophores can be used, which, upon irradiation with light (also designated as pump light) of one color, for their part emit light of a different color, in other words convert light. In this way, white light can be generated for example by irradiating phosphor with blue light.
In the case of light-conversion-based lamps that are customary nowadays, for example a planar phosphor-containing layer is illuminated from below with blue light, which is usually generated by means of LEDs. Here the light is incident perpendicularly to the phosphor-containing layer, which for its part generates white light in accordance with a Lambertian emission characteristic.
With the use of such an arrangement in which a material (or luminophore) that emits light diffusely is irradiated by means of a light source, the light emission from the luminophore takes place substantially in accordance with a Lambertian distribution. That is to say that, in the case of a surface element which is coated with diffusely reflective material and/or a phosphorescent material and/or a fluorescent material and which is illuminated by a light source, light is emitted predominantly perpendicularly to the surface element, wherein the light emission decreases continuously toward the sides.
If such an arrangement is part of an optical system, then this effect just described can have the disadvantage that the converted light possibly does not impinge on an optical unit and, as a result, is lost for further utilization. In general, a targeted variation of the light emission characteristic may be desired, for example a widening of the light distribution in reflector applications, or a narrowing of the light distribution in lens optical units.
Problems can also occur on account of the fact that customary luminophores do not completely convert the incident light, but rather also always diffusely reflect part thereof. Firstly, this scattered light is then no longer available to the conversion process and reduces the degree of conversion. Secondly, the presence of the pump light having a specific color can have a disadvantageous effect in applications where full conversion into a different color is desirable, for example in projection applications, where color fidelity is of fundamental importance.
In the irradiation of luminophores, the limitation is furthermore present that said luminophores cannot be irradiated with arbitrary pump light powers. Excessively high power densities on the luminophore can impair the functioning thereof or even damage the luminophore.